Numerous studies in the last ten years have demonstrated the key role of melatonin (N-acetyl-5-methoxytryptamine) in many physiopathological phenomena and in the control of circadian rhythm. Its half-life is quite short, however, owing to the fact that it is rapidly metabolised. Great interest therefore lies in the possibility of providing the clinician with melatonin analogues that are metabolically more stable, have an agonist or antagonist character and may be expected to have a therapeutic effect that is superior to that of the hormone itself.
In addition to their beneficial action on circadian rhythm disorders (J. Neurosurg. 1985, 63, pp. 321-341) and sleep disorders (Psychopharmacology, 1990, 100, pp. 222-226), ligands of the melatoninergic system have valuable pharmacological properties in respect of the central nervous system, especially anxiolytic and antipsychotic properties (Neuropharmacology of Pineal Secretions, 1990, 8 (3-4), pp. 264-272) and analgesic properties (Pharmacopsychiat., 1987, 20, pp. 222-223), as well as for the treatment of Parkinson's disease (J. Neurosurg. 1985, 63, pp. 321-341) and Alzheimer's disease (Brain Research, 1990, 528, pp. 170-174). Those compounds have also demonstrated activity in relation to certain cancers (Melatonin--Clinical Perspectives, Oxford University Press, 1988, pp. 164-165), ovulation (Science 1987, 227, pp. 714-720), diabetes (Clinical Endocrinology, 1986, 24, pp. 359-364), and in the treatment of obesity (International Journal of Eating Disorders, 1996, 20(4), pp. 443-446).
Those various effects are exerted via the intermediary of specific melatonin receptors. Molecular biology studies have demonstrated the existence of a number of receptor sub-types that are capable of binding that hormone (Trends Pharmacol. Sci., 1995, 16, p. 50; WO 97,04094). It has been possible, for various species, including mammals, for some of those receptors to be located and characterised. In order to be able to understand the physiological functions of those receptors better, it is of great advantage to have available specific ligands. Moreover such compounds, by interacting selectively with one or other of those receptors, may be excellent medicaments for the clinician in the treatment of pathologies associated with the melatoninergic system, some of which have been mentioned above.
The compounds of the present invention have a novel structure characterised by a heterocyclic ring structure that is substituted by an alkylamide chain and that in addition has two substituents on the main ring structure. Surprisingly, that structure confers on them a very high affinity for melatonin receptors and a selectivity for one or other of the receptor sub-types.